As is known, metal guardrails normally comprise a number of vertical supporting posts fixed into the ground one after the other along the edge of a road; and a number of transverse horizontal retaining members fixed to the vertical supporting posts one after the other to form a straight longitudinal retaining strip extending continuously along the edge of the road at a given height off the ground.
When the guardrail starts at a fork in the road or alongside particularly hazardous obstacles, the start end of the guardrail is normally covered with safety structures for retaining or redirecting vehicles towards the centre of the road, depending on whether the impact trajectory of the vehicle against the end of the guardrail is tangent or not to the guardrail, and with deceleration obviously below current regulation thresholds.
Safety structures for the above purpose are normally referred to as “guardrail terminals”, and normally comprise a reinforced-concrete base at ground level; a number of vertical supporting posts arranged successively in a U on the reinforced-concrete base, starting from the end of the guardrail; a number of programmed-yield anchoring bolts for securing each vertical supporting post firmly to the reinforced-concrete base; and a number of collapsible horizontal longitudinal members fixed telescopically one after the other to the vertical supporting posts to form a collapsible, substantially horseshoe-shaped horizontal beam, i.e. U-shaped in a horizontal plane.
Another commonly used type of guardrail terminal comprises a prismatic, triangular-based tank made of plastic material, anchored to the ground immediately upstream from the start end of the guardrail, and filled with water to absorb the impact of the vehicle.
Another type of guardrail terminal comprises a thin metal tubular member fixed vertically and directly to the vertical supporting post at the end of the guardrail.
The first type of guardrail terminal described above has the drawback of being extremely expensive to produce, and of failing to effectively absorb the kinetic energy of the vehicle in collisions involving a vehicle travelling on the other side of the road, i.e. when the vehicle is travelling on the opposite side of the road to the edge bounded by the guardrail, and strikes the rear of the terminal, possibly after scraping against the end/initial portion of the guardrail.
Though decidedly cheaper to produce, the other guardrail terminals described above provide for fairly poor absorption of the kinetic energy of the vehicle, and are therefore only suitable for installation on slow roads.